Is there a document on the Glide’s website?
There is a lot to learn from the Glide library. I don’t know of a specific page about performance, but there might be.
Do I avoid relations as much as possible?
I think you can trust relations in Glide Tables to be speedy.
Should I only work with user rows?
I don’t know about “only”. You can use “row owner” if you want the owner of that row only to see the data in that row and nobody else ever. Using “row owners” and “roles” rather than filtering and visibility will speed up your app if you have a lot of rows (25k, 50k, 100k …).
A few ideas here: